The statements in this section merely provide background information related to the present disclosure. Accordingly, such statements are not intended to constitute an admission of prior art.
Internal combustion engines, especially automotive internal combustion engines, generally fall into one of two categories, spark ignition engines and compression ignition engines. Traditional spark ignition engines, such as gasoline engines, function by introducing a fuel/air mixture into the combustion cylinders, which is then compressed in the compression stroke and ignited by a spark plug. Traditional compression ignition engines, such as diesel engines, typically function by introducing or injecting pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke, which ignites upon injection. Combustion for both traditional gasoline engines and diesel engines involves premixed or diffusion flames that are controlled by fluid mechanics. Each type of engine has advantages and disadvantages. In general, gasoline engines produce fewer emissions but are less efficient, while, in general, diesel engines are more efficient but produce more emissions.
More recently, other types of combustion methodologies have been introduced for internal combustion engines. One of these combustion concepts is known in the art as the homogeneous charge compression ignition (HCCI). The HCCI combustion mode includes a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry, rather than by fluid mechanics. In a typical engine operating in HCCI combustion mode, the cylinder charge is nearly homogeneous in composition temperature at intake valve closing time. Because auto-ignition is a distributed kinetically-controlled combustion process, the engine operates at a very dilute fuel/air mixture (i.e., lean of a fuel/air stoichiometric point) and has a relatively low peak combustion temperature, thus forming extremely low nitrous oxides (NOx) emissions. The fuel/air mixture for auto-ignition is relatively homogeneous, as compared to the stratified fuel/air combustion mixtures used in diesel engines, and, therefore, the rich zones that form smoke and particulate emissions in diesel engines are substantially eliminated. Because of this very dilute fuel/air mixture, an engine operating in the auto-ignition combustion mode can operate unthrottled to achieve diesel-like fuel economy. The HCCI engine can also operate at stoichiometry with substantial amounts of exhaust gas recirculation (EGR).
There is no direct control of start of combustion for an engine operating in the auto-ignition mode, as the chemical kinetics of the in-cylinder charge composition determine the start and course of the combustion. The chemical kinetics of the in-cylinder charge composition (i.e., in-cylinder oxygen mass) are sensitive to ambient conditions that include intake air temperature, altitude and humidity. As such, these ambient conditions can influence the maximum efficiency and robustness during HCCI operation and when the engine is operating at stoichiometry with substantial amounts of EGR. It is known, for example, that as humidity increases, a portion of oxygen and nitrogen in the mass airflow is replaced by water vapor leaving less oxygen available within the in-cylinder charge composition for combustion. In other words, the in-cylinder oxygen mass decreases with increasing humidity where the chemical kinetic reaction for auto-ignition would slow down due to the effect of water vapor. It is further known, for example, that as the altitude changes, the in-cylinder oxygen mass can vary. For instance, as altitude increases the in-cylinder oxygen mass decreases.